The instant invention relates to fiber optic transmission cables and more particularly to a reinforcing element for reducing the susceptibility of a fiber optic transmission cable to physical damage, such as thermal stresses or stresses caused during installation procedures.
It has generally been found that it is necessary to provide some means for reinforcing optical glass fibers when they are used in applications wherein they are likely to be exposed to tensile and/or sheer stresses. In this regard, while flawless glass fibers generally have relatively high tensile strengths, most glass fibers, such as those used in fiber optic transmission cables, have at least some minor physical defects which are known to propagate under mechanical stresses, substantially reducing their tensile strengths and rendering them prone to mechanical failure. Since fiber optic cables are often exposed to substantial tensile stresses when they are installed in conduits, some means for reinforcing the fibers of fiber optic cables is generally necessary to prevent them from fracturing during installation procedures for both underground and overhead applications. In addition, since the optical fibers in fiber optic cables are also likely to be exposed to both tensile and compressive forces during normal use due to variations in the thermal expansion coefficients of the various components of the fiber optic cables in which they are assembled, some means is generally necessary for reinforcing the fibers of fiber optic cables during normal use as well.
Accordingly, for the above reasons, most fiber optic transmission cables generally include strength reinforcing elements for minimizing the stresses on the optical glass fibers thereof. Such strength elements are typically constructed of materials having high tensile strengths, and high Young's moduli, but which are nevertheless relatively flexible, such as fiber reinforced resin materials or steel. In this regard, however, since electrically conductive strength elements are prone to attracting atmospheric lightning discharges when they are used in aerial installations, it has been found that electrically nonconductive strength elements are preferable for aerial applications. Further, nonconductive strength elements have also been found to be preferable for applications such as in explosive areas or in areas of high electromagnetic interference, for example in electrical power distribution corridors.
A variety of different fiber optic cable constructions which have comprised strength elements of various types have been heretofore available. In this regard, typically, most strength elements have been formed as elongated wire-like elements of substantially circular cross section and they have been used in cables in combination with various fiber supporting structures. Strength elements of this type have generally been assembled with various casings or outer jackets to form fiber optic cables, wherein the optical fibers thereof are positioned in predetermined orientations by the fiber supporting structures thereof, and protected against excessive stresses by the strength elements thereof. The fiber optic cables disclosed in the U.S. Pat. No. 4,038,489 to Stenson et al; King et al U.S. Pat. No. 4,154,049; Oestreich U.S. Pat. No. 4,199,224; Yonecki U.S. Pat. No. 4,235,511; Arnaud U.S. Pat. No. 4,354,732; Williams U.S. Pat. No. 4,361,381; Trezequet U.S. Pat. No. 4,389,088; Hope U.S. Pat. No. 4,401,366; Le Noane et al U.S. Pat. No. 4,408,828; Smith U.S. Pat. No. 4,435,238; Whitehead et al #4,456,331; Yataki U.S. Pat. No. 4,474,426 are exemplary in this regard and all comprise elongated strength elements of substantially circular cross section, elongated fiber supporting structures and outer sheaths or casings. In addition, many of the cables disclosed in the above references are constructed so that the fiber supporting structures thereof are extruded over the strength elements thereof to provide longitudinal channels for receiving optical fibers therein and many of the cables disclosed in references include various waterproof gels for protecting the optical fibers thereof against water damage and freezing. The U.S. Pat. No. 4,227,770 to Gunn; Nakagome et al U.S. Pat. No. 4,257,675 and Trezeguet et al U.S. Pat. No. 4,422,889 disclose various other fiber optic cable constructions, some of which comprise strength elements of noncircular sectional configuration. However, while the various references hereinabove cited represent the closest prior art to the instant invention of which the applicant is aware, they fail to disclose or suggest, a strength element for a fiber otpic cable which embodies the novel structural features of the strength element of the instant invention and hence, these references are believed to be of only general interest. More specifically in this regard, the above references, fail to disclose or suggest a strength element which is integrally formed from a fiber reinforced resin material and which has the novel and unobvious sectional configuration of the strength element of the instant invention as will hereinafter be made apparent.
The instant invention provides an effective strength element for a fiber optic cable which has substantial advantages over the heretofore available strength elements. In particular, the instant invention provides a strength element which can be effectively utilized for constructing a fiber optic cable having a signficantly reduced sectional dimension without reducing the transmission capacity of the cable. In this regard, the strength element of the instant invention comprises an elongated central portion and at least three substantially longitudinally extending ribs which extend outwardly from the central portion to define substantially longitunally extending grooves therebetween in the strength element. The strength element is integrally formed from a fiber reinforced resin material and the ribs are preferably formed so that they extend substantially radially outwardly from the central portion and so that they are tapered in their outward extents from the central portion. More specifically, the ribs are preferably formed so that the opposite sides of each thereof define an angle therebetween of between approximately 6.degree. and 70.degree.. The ribs are preferably further formed so that the outer extremities thereof are of rounded configuration and so that the inner extremities of the grooves which are formed between the ribs are also of rounded configuration. In this regard, the grooves between the ribs are preferably formed so that the radii of curvature thereof are between approximately 1 and 1/20 of the outward extents of the adjacent ribs from the central portion. Further, the strength element is are preferably formed so that the ribs thereof twist or spiral around the central portion in their longitudinal extents in the strength element, or alternatively so that they twist or spiral in periodically reversing directions around the central portion.
It has been found that the strength element of the instant invention can be effectively utilized for constructing a fiber optic cable of substantially reduced diameter and hence, also reduced cost. Specifically, because the strength element of the instant invention is integrally formed of a fiber reinforced resin material in the configuration hereinabove specified, it can function as both a strength element for reinforcing the optical fibers in a cable and as a supporting structure for receiving and positioning the optical fibers, and that therefore a cable constructed therewith can be made in a reduced overall maximum sectional dimension. In this regard, because of the configuration of the strength element of the instant invention, when it is assembled with optical fibers for use in a cable so that the fibers are received in the grooves in the strength element, the overall maximum sectional dimensions of the assembly comprising the strength element and the fibers is substantially the same as the maximum sectional dimension of the strength element by itself. On the other hand, when a conventional strength element of substantially circular cross section, but substantially the same maximum overall sectional dimension, is assembled with optical fibers for use in a cable so that the fibers are disposed around the circumference of the strength element, the assembly comprising the conventional strength element and the optical fibers has a maximum sectional dimension which is substantially greater than the maximum sectional dimension of conventional strength element by itself. Hence, the overall sectional dimension of a cable constructed with the strength element of the instant invention is substantially less than that of a conventional fiber optic cable of similar transmission capacity and as a result substantial savings in material costs are realized. Further, it has been found that the specific sectional configuration of the strength element of the instant invention herein specified provides effective resistance to breakage during the type of bending thereof which is likely to occur during cable installation procedures and/or during normal use. More specifically, it has been found that the outwardly tapered configurations of the ribs, the rounded terminal ends of the ribs, and the rounded configurations of the inner extremities of the grooves between the ribs substantially increase the resistance of the strength element to breakage during bending as compared to a strength element having longitudinally extending ribs and grooves which do not embody these features. Further, it has been found that the strength element provides substantial resistance to the other forces to which optical fibers are exposed during normal use, such as, those resulting from the differences in the thermal expansion coefficients of various cable components. Hence, it is seen that for the above reasons, the strength element of the instant invention can be effectively utilizied for manufacturing fiber optic cables of substantially reduced sectional dimension. As a result, since a greater number of cables can be installed in a particular conduit when the cables have reduced sectional dimensions, the transmission capacity of the conduit can be effectively increased by utilizing cables constructed with the strength element of the instant invention.
Accordingly, it a primary object of the instant invention to provide an effective strength element for a fiber optic cable which allows the cable to be constructed with a reduced sectional dimension.
Another object of the instant invention is to provide a strength element which is made of a fiber reinforced resin material and which includes a plurality of substantially longitudinally extending ribs thereon which define grooves therebetween for receiving optical fibers.
An even further object of the instant invention is to provide a strength element which can be utilized for constructing fiber otpic cables having reduced material costs.
A still further object of the instant invention is to provide a strength element for a fiber optic cable which is made of a fiber reinforced resin material and which includes a plurality of substantially longitudinally extending ribs which define grooves therebetween for receiving optical fibers, wherein the strength element has increased resistance to breakage during bending.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.